Not only is high flow rate control accuracy required for a flow rate control apparatus used with semiconductor manufacturing facilities, and the like, but also a considerably wide control range is required with regard to the flow rate control range. As the required flow rate control range becomes greater, it is inevitable that control accuracy is lowered in low flow rate situations. Thus, it is difficult to make up for the degradation of control accuracy in a low flow rate state using only a flow rate control apparatus provided with a feature with which to correct a measured value. To overcome this problem in a general way, a flow rate control range may be divided into a plurality of flow rate areas, e.g. the area for a large flow quantity, the area for a medium flow quantity and the area for a small flow quantity, in order to meet a required flow rate control range. This solution, however, involves providing 3 sets of flow rate control apparatuses, each one responsible for the flow rate control of one of each one of the flow rate areas in parallel so that high flow rate control accuracy can be maintained over a wide flow rate control range.
However, in a system, in which a plurality of devices responsible for different flow rate control ranges, respectively, are provided in parallel, installation costs unavoidably go up, which makes it difficult to reduce the installation costs. At the same time, switching operations of flow rate control apparatuses become time-consuming and troublesome. Also, with respect to semiconductor manufacturing facilities, it has become more popular these days to replace the conventional thermal type mass flow rate control apparatus with a pressure type flow rate control apparatus. The reason for this replacement is that a pressure type flow rate control apparatus is not only simple in structure, but also has excellent properties with respect to responsiveness, control accuracy, control stabilities, manufacturing costs, maintainability, and the like. Furthermore, a flow rate control apparatus can be easily replaced with a thermal type mass flow rate control apparatus.
FIG. 7(a) and FIG. 7(b) illustrate one example of the basic structure of the afore-mentioned conventional pressure type flow rate control apparatus FCS. A major portion of the pressure type flow rate control apparatus FCS comprises a control valve 2, pressure detectors 6, 27, an orifice 8, flow rate computation circuits 13, 31, a flow rate setting circuit 14, a computation control circuit 16, a flow rate output circuit 12, and the like.
In FIG. 7(a) and FIG. 7(b), 3 designates an orifice upstream side pipe; 4 designates a valve driving part; 5 designates an orifice downstream side pipe; 9 designates a valve; 15 designates a flow rate conversion circuit; 10, 11, 22, 28 designate amplifiers; 7 designates a temperature detector; 17, 18, 29 designate A/D converters; 19 designates a temperature correction circuit; 20, 30 designate computation circuits; 21 designates a comparison circuit; Qc designates a computation flow rate signal; Qf designates a switching computation flow rate signal; Qe designates a flow rate setting signal; Qo designates a flow rate output signal; Qy designates a flow rate control signal; P1 designates orifice upstream side gas pressure; P2 designates orifice downstream side gas pressure; and k designates a flow rate conversion rate. The afore-mentioned pressure type flow rate control apparatus FCS shown in FIG. 7(a) is mainly used either in the case where the ratio P2/P1 of the orifice upstream side gas pressure P1 and the orifice downstream side gas pressure P2 is equal to the critical value of a fluid, or in the case where the ratio P2/P1 is lower than the critical value (that is, when a gas flow is constantly under the critical state). The gas flow rate Qc passing through the orifice 8 is given by Qc=KP1 (where K is a proportionality constant).
The afore-mentioned pressure type flow rate control apparatus FCS shown in FIG. 7(b) is mainly used for the flow rate control of gases that will be in the flow condition in both the critical and non-critical states. The flow rate of a gas passing through an orifice is given, in this case, by Qc=KP2m(P1−P2)n (where K is a proportionality constant, m and n are constants).
With the afore-mentioned pressure type flow rate control apparatus in FIG. 7(a) and FIG. 7(b), the setting value of the flow rate is given by a voltage value as Qe, the flow rate setting signal. For example, suppose that the pressure control range 0˜3 (kgf/cm2 abs) of the upstream side pressure P1 is expressed by the voltage range 0˜5V, then Qe=5V (full scale value) becomes equivalent to the flow rate Qc=KP1 at the pressure P1 of 3 (kgf/cm2 abs). For instance, when the conversion rate k of the flow rate conversion circuit 15 is set at 1, a switching computation flow rate signal Qf (Qf=kQc) becomes 5V if the flow rate setting signal Qe=5V is inputted; thus, a control valve 2 is operated for opening and closing until the upstream side pressure P1 becomes 3 (kgf/cm2 abs) in order to allow the gas of flow rate Qc=KP1, corresponding to P1=3 (kgf/cm2 abs), to flow through the orifice 8.
In the case where the pressure range to control is switched to 0˜2 (kgf/cm2 abs), and the pressure range is expressed by a flow rate setting signal Qe of 0˜5(V) (that is, when a full scale value 5V gives 2 (kgf/cm2 abs)), the afore-mentioned flow rate conversion rate k is set at ⅔. As a result, if a flow rate setting signal Qe=5(V) is inputted, the switching computation flow rate signal Qf becomes Qf=5×⅔(V) because of the relationship Qf=kQc. And thus, the control valve 2 is operated for opening and closing until the upstream side pressure P1 becomes 3×⅔=2 (kgf/cm2 abs).
In other words, the full scale flow rate is converted so that Qe=5V expresses a flow rate Qc=KP1 equivalent to P1=2 (kgf/cm2 abs). Under a critical condition, the flow rate Qc of a gas passing through the orifice 8 is given by the afore-mentioned equation Qc=KP1. However, when the type of gas whose flow rate is to be controlled changes, then the afore-mentioned proportionality constant K also changes if the same orifice 8 is in use.
It is also same, in principle, with the afore-mentioned pressure type flow rate control apparatus in FIG. 7(b). The flow rate Qc of a gas passing through the orifice 8 is given by Qc=KP2m(P1−P2)n (where K is a proportionality constant, and m and n are constants). When the type of gas changes, the afore-mentioned proportionality constant K also changes.    [Patent Document 1] TOKU-KAI-HEI No. 8-338546 Public Bulletin    [Patent Document 2] TOKU-KAI No. 2000-66732 Public Bulletin    [Patent Document 3] TOKU-KAI No. 2000-322130 Public Bulletin    [Patent Document 4] TOKU-KAI No. 2003-195948 Public Bulletin    [Patent Document 5] TOKU-KAI No. 2004-199109 Public Bulletin